This invention relates to a method of inspecting the surface of an object for detecting unfavorable foreign matter existent on a surface, and apparatus therefor. More particularly, it relates to a method of detecting foreign matter suitable for the inspection of the external appearance of a surface of a semiconductor wafer, and apparatus therefor.
In the manufacturing steps of a semiconductor device such as formation of a passivation film and formation of a metallic interconnection, foreign matter of an insulating substance, a metallic substance, dust etc. often adheres to undesired areas of the surface of a semiconductor wafer. The foreign matter forms a cause for noticeably lowering the yield or available percentage of the semiconductor device. In the process management, therefore, it is necessary to determine the sizes, number etc. of the foreign matter in advance. Accordingly, an inspection on the foreign matters on the semiconductor wafer is required between the actual manufacturing steps of the semiconductor device.
Heretofore, the inspection on the foreign matter has been usually made by the visual inspection employing a metallurgical microscope (of, for example, 100-200 magnifications). This method has the problem that the inspector's subjectivity is prone to be involved in the judgement of a foreign matter, so the inspected results differ depending upon inspectors, and the problem that a long time is taken for the inspection (2-3 hours for one semiconductor wafer). It is therefore impossible to accurately and promptly offer the result of the interstep inspection and to perform an appropriate process management, so that the lowering of the yield cannot be effected.
Although not relevant to the foreign matter detecting method taken up as the subject of this invention, a method of inspecting the evaporated state of a metal film in a semiconductor device has been proposed in Japanese patent application Publication No. 32539/1976. According to the method, the surface of the semiconductor device is illuminated by a collimated beam, and by exploiting the difference of the directivities of irregularly reflected light from the metal film and irregularly reflected light from an insulating film of the semiconductor device on which the metal film ought to be evaporated, the irregularly reflected light from the evaporated metallic pattern is converted into an electric signal, whereby the metallic pattern is judged to be conformable or defective. Such a method of inspecting the pattern of the metal film is, in the end, achieved for the first time when the different substances of the insulating film such as silicon oxide film and the metal film to be evaporated thereon are combined. In case of intending to apply such a method to the automation of foreign matter detection, problems to be described below are posed. The semiconductor device to be inspected includes, for example, a protuberance of a normal pattern which has been formed by a metallic interconnection layer and an unfavorable protuberance of a foreign matter which has developed due to the step of evaporating a metal. Therefore, when the surface of the semiconductor device is illuminated by the collimated beam and the reflected light is converted into the electric signal, the detected electric signal of the protuberance of the normal pattern becomes a noise component to the detected electric signal of the protuberance of the foreign matter. This results in the difficulty of the discrimination between foreign matter and the normal pattern. Moreover, in the case where a foreign matter has been formed on a substrate made of the same substance as that of the foreign matter, for example, where the normal pattern of a passivation film of phosphosilicate glass formed by the chemical vapor deposition (CVD) process has been overlain by foreign matter of the phosphosilicate glass appearing during the process, the optical properties of the foreign matter and the normal pattern are identical, so that the discrimination of the foreign matter is very difficult.